Boil Volume Question

Maybe I am overthinking this but I am wondering how I determine boil volume. I am getting ready to brew up a batch of Brewers Best Holiday Ale, the directions are laid out for a partial boil, but everything I have read indicates that a full boil is a better route to go. Based on that piece of information, am I looking for 5 gallons going into the boil, 5 gallons out of the boil accounting for loss due to evaporation (this seems correct to me to hit the target gravity), or 5 gallons out of the fermenter accounting for some loss in the trub (more beer into bottles = good. how do you estimate loss in trub)?

Help please, my brain hurts.....

A full boil typically refers to getting the desired batch size post boil. Therefore, if your recipe called for a 5.25 gallon batch, you would boil enough water (say 6.25 gallons for 60 minutes) to net you 5.25 gallons after the boil. I personally don't stress about trub losses because IMHO they are negligilble unless you are making something like a pumpkin ale that has a boatload of trub. Finally, a full boil only helps with hops utilization. If your recipe calls for a partial boil and you do a full boil, then you will end up with a slightly more bitter beer. I suppose you could also say that a full boil helps with piece of mind because all of the water that is used in the final beer is boiled and hence bug free. But that can be cured by boiling the top off water ahead of time, or like many, if your water is good, just pour it in directly to top off. Also, although more beer into the bottles is tempting, just remember if the recipe is designed for a 5 gallon batch and you make a 6 gallon batch post boil, then you will have beer that is 20% diluted unless you compensate by adding more ingredients.

It's all a bit of trial and error really.
I would shoot for about 5.25 to 5.50 gal going into the fermenter if you want to actually get a full 5 gal yield when you bottle.

Generally I would say:
Boil loss about 1 qt every 15 min
Cooling loss about 5%
Also some loss to hops and trub depending on how many hops you have in the recipe. An IPA for example would lose quite a bit to hopping. The size of your pot and how rapid the boil is will also change things a bit.

You can see that you would start with anywhere between 6-7 gal preboil.
Take a crack at it and just have a plan B. You can always add a qt or so of water if you come up short.