It's a nice idea; no doubt, it's less arbitrary. Then, it also discourages being high scoring. Under your recommendation, a 6 point lead at 100 (earn 10 before your opponent earns 16, an obstacle 62.5% as challenging as your opponent's) is more tenuous than a 6 point lead at 50 (earn 5 before your opponent earns 11, an obstacle 45.5% as challenging as your opponent's).
I guess that's your aim. But for now I feel every 4-minute 6-point lead should be rewarded as equal.