Is your caramel neat (ie. like treacle) or is it already broken-down to RFU strength?
From my recall, we used to make a 10% solution (so 100g of neat caramel made up to a litre).
This gave a working solution with a colour around 3000 EBC. Lager is usually around 6-8 EBC, so if your lager is currently very light then I'd look at adding 1ml of the 10% solution per litre of lager.
That will add about 3 EBC to your existing colour. To get a better guide as to addition rates, add 1ml of the working solution (ie. the 10% stuff) to 1 litre of water and compare that to your lager, before adding anything.
Hope that helps!