A naïve assumption is that a handicap stone is worth twice komi. It is easier to translate
proper handicap than traditional handicap, as traditional handicap favors white.
So your traditional 3 stones, no komi translates to about 2.5 ranks, which each rank being twice komi. So 2.5 x 2 x 7 = 35 points.
You could start with that and see how it goes. But the reason I say it's naïve is that it depends on how well the black player uses handicap stones. I recall a Chinese pro saying that handicap stones are easier for the stronger player to neutralize, especially at higher handicaps, but there is obviously no special strategy to neutralize komi.
Also, I strongly suspect that the equivalence, in addition to being non-linear, also varies with strength. See, for example, the
EGF winning statistics data. These show that at, say, EGF 8k a player still has almost a 30% chance of winning a game against a player three stones stronger. By 2d, the chance for the underdog (in this case against a 5d opponent in an even game) drops to 12.6%. Now this isn't handicap data, but my point is that if you try this yourself, you might be surprised and how many handicap games you win or lose vs. an opponent of your "same strength" and how far that handicap can be pushed.