Why bother? Day mode only needs to be brigher than night mode. It is IMPOSSIBLE to calibrate a day mode because light changes all day and when the light changes, so does the image. Light at 8AM is different than the light at 10 or noon or 3PM. Room light is different than sunlight. Sunlight is different on a clear day and a cloudy day. Sunlight is different when there are leaves on the trees versus when the leaves have fallen. Winter light is different than summer light. Sunlight is different when there is snow on the ground versus green or brown grass. There are so many things that change "light" that it's impossible to calibrate for "day".
All you can do is use test patterns and measurements to see how bright you can make the TV without the color shifting or clipping. Say your night calibration is correct (~35 fL for 100% white) with Contrast set to 50. And you determine the highest setting you can use without having problems in the highlight areas is 84. That means when there is light in the room, you can make the picture as bright as you want it to be and it will still be relatively well calibrated as long as you don't set Contrast higher than 84. If that's not bright enough, you'll just have to set Contrast higher and live with any discolorations that might become visible.
And Mitsubishi Diamond WHAT? DLP RPTV? Laser/DLP RPTV? if so, there is a menu item in the User Menu that you have to enable to get access to custom modes (not available for all models, only the top 2 or 3 models). Mitsu front projectors have different menus but you can't really watch a front projector in the "Day" unless you are using a very directional high-gain screen or enclosed RPTV setup with mirrors and a rear projection screen.