Im kinda transferring to odds rather than percentages now but use

outs x 4 for 1 - 8 outs over turn & river

(outs x 4) - 1 for 9 -12 outs over turn and river

(outs x 4) - 4 for 13-16 outs over turn and river

and anything more than that you always have odds to call anyway.

To get odds for just one street you just divide by 2 so

outs x 2 for 1-8
(outs x 2) -1/2 for 9-12
(outs x 2) - 2 for 13 - 16