Surely you did enough simple arithmetic at junior school to know that if there are three things in a formula, and you have any two of them, it's 5 seconds work to rearrange the formula to find the other one?

12volt 12 watt halogen bulb is all you need. Ensure bulb does not touch any combustible surface as it gets quite hot.

Be aware that a 7.5AH battery will be rated for a c/20 amp discharge, 7.5/20 ie 375mA

So if you are discharging at 1 amp which is c/7.5 you will probably get no more than 3AH out before it is completely discharged.

SLA batteries do not appreciate discharging more than 50% (some will say less) otherwise their performance deteriorates further and they must be recharged immediately after discharge, to prevent permanent damage.

If the battery voltage drops below about 10.5V, you absolutely must immediately recharge it to prevent
a large loss in capacity and performance. Lead-acid batteries are very easily ruined by over-discharge, and
should always be stored fully charged and topped up regularly in storage (every month at least).

I'd recommend never discharging below 11V at at all possible.