With a little program I did some quick calculations according to
lichess.org/blog/Wh9KWiQAAI5JrKVn/introducing-rapid-ratings which gave the following results to just reach classical time controls:
'0+37.5', '1+36.0', '2+34.5', '3+33.0', '4+31.5', '5+30.0', '6+28.5', '7+27.0', '8+25.5', '9+24.0', '10+22.5', '11+21.0', '12+19.5', '13+18.0', '14+16.5', '15+15.0', '16+13.5', '17+12.0', '18+10.5', '19+9.0', '20+7.5', '21+6.0', '22+4.5', '23+3.0', '24+1.5', '25+0.0'
You might need to round up according to the options available.
Have Fun!