Dear Ken,
The common sense is to direct 3° backwards from the CG, to the mid point of the leadout. (It is very simple to count, because sin and tg3° is right .05 or in other words: 5%, or 1/20.
Naturally this is a good measure, when you use your fullsize plane on fullsize line with 0.015" dia lines.
Ifyou feel yourself being enforced to use .018 lines, maybe this 3° can be not enough, try more (maybe 4°), as default.
(I am so uncertain, because we in Europe practically never used .018 lines)
Istvan