CAS and small numbers

[parisse]

I would recommend using symbols for physical constants, e.g. h for the Planck constant, then at the end of the computation replace h and other constants by their numerical value, for example
subst(expression,[h,q],[6.626e-34,1.6e-19])
or with units:
mksa(subst(h/m*q,[h,m,q],[_h_,_me_,_qe_]))