The formula depends on the length of your cable. In short:

cable_length * scale_factor = final_cable_length

If the original cable is 108 quants and the scale factor is 2.5, the final cable length, how long it would actually be, is 108*2.5 or 270 quants. It's a very simple formula - at the elementary level. This is also why I suggested using a cable that is much shorter so you have enhanced accuracy.

Edit: with the scale change of 0.018, the scale factor would change by 0.017578125 per tick or 0.28125 per second (16 ticks make one second). Plug this into the formula, a cable at 108 quants will change length by 30.375 quants every second. If you use a 16 quants to 1 foot map scale, this is almost 2 feet per second which is about 1.3 mph. That's rather slow for an elevator, except old ones. For big buildings, about 20 mph is the norm (estimated) which is roughly 15 times faster.