You can put this solution on YOUR website! No!
to keep quantities in proportion you must multiply NOT add.
E.g.
1:2, 2:4, 3:6, 4:8, 5:10, etc. these are all in the same proportion
consider 1:2
if you add 3 to each quantity you get 4:5 which is not in the correct proportion
if you multiply them both by 3 you get 3:6 which is in the same proportion.