Changing stems and estimating new saddle tip to bar "reach"

[pdmtong]

I am trying to figure out how to get more saddle tip-bar length and bar drop from a taller headtube bike currently set up with 30mm spacers under a -6dx100 stem.

Lets say I have my saddle at my desired height and setback.

Now I measure reach from the tip of the saddle to the center of the bar at the interface of the stem and face plate.

If I input the existing stem angle/length along with a new stem angle length into the stem calculator http://yojimg.net/bike/web_tools/stem.php
how close will the approximation of the new saddle tip to bar measure be if I add the existing (which I can measure) with the reach measure delta (27mm) determined by the stem calculator?

current stem 1 is a -6d x 100 with 30mm spacers under it.
proposed stem 2 would be -20d x 120 with 10mm spacers under it.
the bar drop delta would be about 4.4 cm between the two.

I realize the saddle tip to bar is a hypotenuse measure since there is bar drop and the stem calculator reach delta is parallel to the ground so strictly speaking they shouldn't be added together.

how much more than the 27mm spit out by the calculator will the new tip-saddle be? > 5mm? <5mm?