So my guess is that the increase is not 70%, but 66.7% (2/3). I say this because the Accuracy charts use base 3/3 for alterations, so an increase equivalent to two levels would put you at 5/3, or an increase of 2/3 accuracy. This follows the pattern of Reflect essentially doubling Defense (a two stage increase in standard Defense chart), Light Screen doubling Special Defense (a two stage increase in the Special Defense chart), and Tailwind doubles Speed (two stages in the Speed chart). It would follow that Gravity would be the equivalent of a two stage increase in the Accuracy chart (or a two stage decrease in the Evasion chart, but there is exactly no difference because it multiplies with other Evasion / Accuracy modifiers). It's most likely a decrease in Evasion because it is a field effect on all Pokemon, but it's less confusing to state it as an increase in Accuracy, and has no testable difference.
If we're talking about Accuracy, how do OHKO's Accuracy and Pokemon levels interact?
Sing (55%):
55/60 = 2/3 increase (OmegaDonut)
15/16 = 70.5% increase (Wizard of Odd)
70/76 = 67.5% increase (total, and the closest amount of hits to a 2/3 increase possible (expected 69 and 2/3 hits = 70 hits)
Dynamicpunch and Zap Cannon (50%)
34/40 = 70% increase (Peterko)
23/27 = 70.4% increase (Wizard of Odd)
57/67 = 70.1% increase (total, and one more than the closest amount of hits to a 2/3 increase (expected 55.8 hits = 56 hits)