I have a 55gal tank - it's 48" long, 20" high and 12" deep. I have a
stanard one-bulb hood on it - the kind where there are 2 lids you can lift
at the front to feed the fish. Right now I have one 40-watt bulb. Is there
some formula for figuring out wattage ? Can I buy a 75-watt or a 100-watt
bulb for this one-bulb hood ? Do I need a new hood ?

I dont know about your current hood but you need minimum two more
40watt bulbs. This will give you little more than 2 watts per gal. Or
go for 2x55w bulbs.

Regards
Dominic