Wingco,
You have the right formula but a wrong twist on it, methinks, unless I have misinterpreted you. (Been known to happen after a claret or two.)
I"ll make up numbers for easy math, but the principle will be the same..
Say your lappy's designed to run on 15 volts, and say that when given 15 volts it draws 5 amps. Using V=IR, then R = 3 ohms. Ie, the effective resistance of your lappy is 3 ohms. This is an oversimplification, but it will do for now.
Now, that resistance is fixed. The lappy will always be 3 ohms.
If you now apply 6 volts, then using V=IR, 6=I x 3. That is I, the current with a lower voltage of 6, will be 2 amps.
A HIGHER voltage than the design 15 will push a higher current in the theoretical lappy, so don't go there.
A LOWER voltage is safe - it can only result in less current.
Cheers
AA
AA