It just depends on the type of boots you want to roll.
You want an affix level of 37.
With Demonhide boots (as an example), which are Quality level 36 (see :
http://classic.battle.net/diablo2exp/items/exceptional/boots.shtml ), you need an item level of 55.
55 - 37/2 = 55 - 18 = 37
Now to get an item level of 55, just use a char level 83 to roll.
Suppose your initial rare demonhide boots have iLvl 99 (you found them from hell Baal).
First roll : 0.4 x 99 + 0.4 x 83 = 39 + 33 = 72
Second roll : 0.4 x 72 + 0.4 x 83 = 28 + 33 = 61
Third roll : 0.4 x 61 + 0.4 x 83 = 24 + 33 = 57
Fourth roll : 0.4 x 57 + 0.4 x 83 = 22 + 33 = 55
Fifth roll and all following rolls : 0.4 x 55 + 0.4 x 83 = 22 + 33 = 55
Notice that, since all numbers are rounded down in the process, you can as well use char level 84.