Autofill referencing every nth cell

Originally Posted by Bo_Ry

Please try at I2
=IF(MOD(ROWS(I$3:I3)-1,I$2),"",$B2+INDEX($B3:$B24,I$2)+($B2+INDEX($B3:$B24,I$2))^0.5)

Hi Bo_Ry, appreciate your input. I am afraid I do not understand how your formula works but it does produce the correct values!
May I please ask how I can modify your formula for N=3 or N=10 for example?

Thank you.