It gets a lot easier if you don't use the arc length and then another calculation back to the change in two dimensions.
The change will occur on an arc, but for low angles, and for something like leveling which isn't super critical, I would just use regular two dimensional right angle trigonometry
Doing it that way you use the conversion of .017" of change vertically for every inch of horizontal distance for every degree which is pretty close for small angles.
So you get for the 79" width
3degrees X .017" X 79 = 4.029"
If I do calc off of the link example, they got 5.026" for 3* at 96"
Doing calc as I did it would 3 X .017 X 96 = 4.896
That is within .13" or 1/8" and you level in 1" increments and have suspension and sway bars mucking up the movement besides so insignificant in the real world.
The very digital levels are accurate to about +/- .03 degrees or so, but the typical ones you buy for under $50 are somewhere in the +/- .2 to .3 degrees, so you have about 10% either way for 3% of error which is nearly 1/2" at you width and 3%. Much more than the calc estimation error.
I always remind users that the digital measuring tools are usually nowhere near the accuracy they display inferred by the number of digits. Many levels display two digits after the decimal point but are only accurate to .3 of one digit. Digital calipers commonly display to 3 digits after the decimal and sometimes even 4 digits and many of them are only accurate to +/- .002" or so. The digital stuff does tend to repeat measurements a lot better than the accuracy so the +/- .3 degrees could easily repeat to .1 degree or better on the same measurement.
The easy way to check a digital level, or bubble level, is on a fairly level surface. Check it with the level in one place and note the reading, turn the level around 180 degrees in and measure the exact same point (I mark with tape) and measure it again. Only the sign + or - of the angle should change and any difference is error times 2 plus placement error.
Booster, you are s genius and the real booster!
1000 thanks!