I think the ideal crossover is as much room and equipment (receiver / pre-pro) as it is speakers. I wrote all of the following to illustrate that a combination of equipment, speakers, room and seating location can cause what you would otherwise think should be the best choice, in reality is not the optimal choice. So answering your in theory question may not be optimal but a rull of thumb is to use a crossover setting that is twice the -3db point of your mains. So the numbers you posted would indicate that a 110 to 120 hz setting would be a very good place to start.
I have a Sony 4ES receiver and old acoustic suspension sansui speakers with 15" woofs. These aren't bookshelve speakers obviously but given the right situation I think some book shelf speakers could cause a similar situation as large speakers. I found it difficult by ear to judge which crossover setting was optimal. I went back and forth between 40 and 100 hz. I adjusted phase as necessary and recorded 1 /12 octave graphs (10 hz - 100 hz) of the different settings. I tried about everything I could think of including mains set to large + sub, Large only (to determine the in room extension capabilities) and small + sub.
I have been tinkering off and on for the past 9 months trying to get an optimal crossover and phase setting for my room. It really wasn't until I viewed my large + sub graph a little harder that I realized I should go beyond conventional wisdom and try something rather extreme. That is, I ran the crossover on the Sony all the way to 200 Hz which is the maximum setting the receiver will allow.
This setting actually produced the best graphs. Because my test tones stop at 100, I cross-referenced 100 hz to 500 hz with 1/3 octave pink noise to make sure nothing was out of whack up high. The pink noise test checked out fine and believe it or not I can not localize the sub. I also changed the set-up back to a conventional 80 hz crossover + sub and ran the pink noise up to 500 and it was basically the same as the 200 hz crossover setting.
The sub sits inches to the right of my front main speaker. I tried this setting with action DVDs and music at loud levels and found nothing unusual. If I ever get where I can localize the sub I'll try something else but so far this seems fine to me.
Why I think this works for me and my room? 1) The Sony seems to have a shallow crossover slope, probably 6 dbs an octave so starting at 200hz works a little better assuming the sub can't be localized. 2) Since the slope is shallow I was getting some minor (.5 db) to major (3db) cancellations below the crossover point between 35 - 75 hz. when using an 80 hz crossover point and the main speakers set to small 3) The main speakers have enough low end out-put to create cancellation and the slope was not getting them out of the way fast enough. 4) Sound quality wise, the sub is a much better bass producer than my mains.
The large + sub combo graph looked pretty good from 40 to 100 hz and would be okay with typical music but below 40 hz I got anywhere from 5 to 12 dbs of cancellation all the way to 17 hz. I made various phase adjustments to no avail.
What I would recommend is that you experiment as much as your internal crossover will allow with different crossovers and don't get caught up in the actual crossover hz setting. Most other receivers have a 12 db per octave slope and if that's the case for you then an 80 - 100 hz crossover may work as the speakers should be sufficiently out of the subs way beyond 40 hz. You will need an SPL meter and some test tones to quantify the results. Generally speaking, most sub woofers are better bass producers than the woofs in speaker cabinets, therefore the higher the setting you can go without localizing the sub the better off you may be. The crossover setting that causes the least disruption in frequency response while not being able to localize your sub will be your best setting.
Many audiophiles like to use a low crossover like 40 hz. Many of these individuals have a 24 db per octave sloped crossover so they can probably pull it off provided their main speakers can go flat to 40 hz. Using a low crossover eliminates the disruption in frequency response that can occur in the critical bass music frequency range when using an 80 hz crossover. Since I don't have a 24 db per octave crossover I went the other direction. Mentally I would prefer the traditional 80-100 hz setting but the reality of it is that the 200 hz setting graphs better and sounds fine to my ears so I will keep with using it until I find something that works better.