I am trying to work out what is the max lighting I can run on the front of my bug for nite driving? I have a just rebuilt alt (55amp)and a brand new battery. I remember a old rule that I think was 10w = 1amp (is that rite?) so how much can I run? 400w worth?

Power = Volts x Amps

or

Amps = Power / Volts

12 W / 12 V = 1 amp

400/12=33 amps

You would want to switch that through a relay and have a pretty thick wire feeding it.

going to do that, run a nice think cable foward , with a relay control before splitting it at the block. Its for lighting that will be removed and refittied when needed. How much does the negine take eg could I run 500w? I already have headlights with 100w globes.

just want to run some spoties for nav runs, and also for going for blasts at nite in the hills.

you should be able to run high wattage lighting as accessories, but am not sure how high... long and tedious way is to measure the current drawn on each circuit and add them up. This will tell you what you have left. Important to ensure you have the correct fusing and cable sizing also. Note that fuses are not there to protect the equipment but there for the cable.
Happy spotlighting.