the height of the M+1 peak in a mass spectrum is proportional to the number of Carbons and the number of nitrogens in the molecular formula. 99% of N is N-14, and 0.37% is N-15. 98.9 % of C is C-12 and 1.1% is C-13. So the height of the M+1 peak will equal the height of the M+ peak times (0.011 times the mumber of carbon atoms plus 0.0037 times the number of nitrogen atoms)
