The following work shall help you. 

Determinations of total carotenoids and chlorophylls a and b ...

www-06.all-portland.net/bst/011/0591/0110591.pdf

by K HARTMUT 

Various equations for the determinations of total chlorophyll and individual ... relative specific absorption coefficients to these values were established ... acetone, 80% (v/v) acetone, 96% (v/v) ethanol and methanol. The red ... values at 470nm for total leaf carotenoids [xanthophyll (x)plus carotenes (c)] in direrent solvents.

Thanks a lot by the reference!

It's hard to find the specific absorbance coefficient for chlorophyll c in methanol at 470 nm. In Fucales or in Brown algae, different authors estimates carotenoids using chlorophyll a and b equations, but I think that it's wrong... I'll study this topic and try to propose the solution.

I'll study this problem. Chlorophyll c you mean carotenoids  ?

Iif so that methanol pure solvent:

Ca=16.72A665,2 - 9.16A652.4   (ug/ml)

Cb=36.92A652.4 - 15.28A665,2

Cc= (1000A470  - 1.63Ca - 104.96 Cb )/221

This method is of book Methods... Now I do not remember exactly.

If you need more accurate method to 80% methanol and a book title then write.