I am wondering how drastically these settings affect things - am I to assume each +1/-1 of either of these values will have profound effect? For instance, I am changing 42/30 to 37/34 in order to increase # of Runs (I want my league to be more balanced Run/Pass than modern NFL) and to have teams stick to the percentages more closely (I am seeing too many runs on 3rd and Long, for instance). Will my changes have a moderate affect on the league or am I underestimating just how strong these figures change things?

Those numbers directly affect run pass. So if you increase both numbers you'll get more passes overall. If you change one number but not the other you're adjusting the affect of play calling. Right now at 42/30 if the play calling has 100% set you'll get a max pass percetage of 72 (for a 72/28 split). If you change it to 37/34 you'll get a max pass of 71 for a 71/29 pass run split. Since you moved both numbers at the same time it won't have much effect since the CPU targets a more modern run pass split of 55/45 by default.

If you want to make run/pass get closer to 50/50 you would just lower the base number... maybe try 37/30 which should drop the ratio a few percent.