Somewhere from faq thread:

UPGRADING SR SKILL LV

1 to 2 - 1x R
2 to 3 - 2x R
3 to 4 - 3x R
4 to 5 - 4x R OR 1x SRs2
5 to 6 - 5x R OR 1x SRs2
6 to 7 - 1x SRs2
7 to 8 - 1x SRs2
8 to 9 - 1x SRs2 + 1x R
9 to 10 - 1x SRs3
10 to 11 - 1x SRs3
11 to 12 - 1x SRs4
12 to 13 - 1x SRs4
13 to 14 - 1x SRs4
14 to 15 - 1x SRs4
15 to 16 - 1x SRs4 + 1x R OR 1x SRGrails3
16 to 17 - 1x SRs4 + 2x R OR 1x SRGrails3 + 1x R
17 to 18 - 1x SRs4 + 3x R OR 1x SRGrails3 + 2x R
18 to 19 - 1x SRGrails4
19 to 20 - 1x SRGrails4
In other words grail is skill fodder, quite potent.