I wrote a small script to calculate the percentages reduced from damage involving Resistances.
Elemental damage reduction = Resistance / (5 × Monster Level + Resistance)
Res lvl 60 lvl 61 lvl 62 lvl 63
50 14,29% 14,08% 13,89% 13,70%
100 25,00% 24,69% 24,39% 24,10%
150 33,33% 32,97% 32,61% 32,26%
200 40,00% 39,60% 39,22% 38,83%
250 45,45% 45,05% 44,64% 44,25%
300 50,00% 49,59% 49,18% 48,78%
350 53,85% 53,44% 53,03% 52,63%
400 57,14% 56,74% 56,34% 55,94%
450 60,00% 59,60% 59,21% 58,82%
500 62,50% 62,11% 61,73% 61,35%
550 64,71% 64,33% 63,95% 63,58%
600 66,67% 66,30% 65,93% 65,57%
650 68,42% 68,06% 67,71% 67,36%
700 70,00% 69,65% 69,31% 68,97%
750 71,43% 71,09% 70,75% 70,42%
800 72,73% 72,40% 72,07% 71,75%
850 73,91% 73,59% 73,28% 72,96%
900 75,00% 74,69% 74,38% 74,07%
950 76,00% 75,70% 75,40% 75,10%
1000 76,92% 76,63% 76,34% 76,05%
With this, you should be able to find your comfort zone with resistances without worrying about putting too many points in.