The gage is actually measuring the distance from the case shoulder to the bullet ogive. You need to compare the shoulders of your own cases to the shoulder of the modified case.
If your case shoulders are .005 longer than the modified case, you would add that .005 to the measured length to get the length that would touch the rifling in your gun.
THEN subtract the .040 or whatever length you choose.
3.508 seems awfully long considering that it would be .168 longer than SAAMI max but only you can know what you gun measures. If it fits and functions, start at starting loads and work up.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.