1d20+40

A small note about a pet peeve of ours, game designers. Please don't 'blow out' your dice range with huge modifiers, it's just crazy. When you start adding modifiers higher than the highest roll of your dice, your original curve or dice range becomes meaningless. It is like rolling 1d6+100, sure it is a valid way to generate 101-106, but the random part of that roll is insignificant compared to the magnitude of the number.

SBRPG's highest negative modifier was about -30 on a 3d6 scale, so we come close. Consider you are adding up to +20 for level, and another +10 for ability DRMs, and you are at about 50% for the best of the best versus an impossible challenge. Good games are good math, but we did edge towards blowing out our 3-18 range at the high levels. At low levels, the modifiers are competitive and fun, and it made for a good game.

I think back to the old DnD3 "Epic Level Handbook" where skills capped out at a +100 modifier on a d20 scale, with difficulties for all sorts of craziness, like walking on air, surviving in a vacuum, and all other sorts of stuff that just goes way outside the realm of traditional fantasy, and would be better handled in a superhero game. The modifiers blew out the maximum roll by five times, and it is not too difficult to get skills past +20 in DnD3.

A good limit is half your total dice range, say a maximum of +10 on a d20 roll. You can go up to the maximum number rolled, say +20, but this should only be for the edge cases. Any modifier higher than what you can roll on the dice makes the roll less significant, and the modifier more important. Luck and the dice marginalized, and it becomes more about stacking numbers.