I am not exactly sure what it is your asking.
I think I understand it but it is a bit confusing how you worded it.
You went from .5 to 1 mil and it is 50%, so in effect whatever the yardage was at .5 it is double at 1 mil, conversly if it is .75 of a mil then it will be double at 1.5 mils. The constant here is the 50% not 75%.
I hate Millradians, thats why I switched over to MOA real easy formula. Here's a link explaining it in detail and take notice of how long it is and how indepth it goes Mil-Dot
, now heres a MOA formula; target size in inches (not a percentage of a yard represented in a decimal form) devided by number of moa lines taken up or dots times 100 = distance. 20/6=3.3X100=330 yards.