Advice with first all grain finished

mashed 8 lbs grain in 10 gallon tun. 3 gallons for mash and 3 gallons for sparge. i was able to get 5 gallons of wort.

i topped of brew kettle to 7.5 gallons, boiled for 90 minutes and ended up with about 4.5 gallons.

i topped it off in the fermentor, but need to know what to do in the future with the evaporation. Should i add more water to the brewpot or more water to the mash, sparge?

I'm supposed to end up with 5 gallons in the fermentor, right?

The typical catch-all measurements would be 1.25 qt per lb of grain for mashin and 2 qts of water per lb for sparging. With 10 lbs of grain that would give you 9.375 gallons of water. You won't get all of that in your brew pot due to the grain soaking some of it up. if you know your boiloff numbers, you can fill your brewpot to just the amount that you need pre boiloff.

You needed more mash and sparge water to start with. You should rinse every bit of sugar from the grain...not top off to boil volume with plain water.

you can do anywhere from 1.25-1.5qt per lb for the mash. sparge with as much water as you need to to get to a good pre-boil volume. If you're not fly sparging you don't have to worry too much about pH and the gravity of your final runnings.

My last grist bill was 9 lbs and I mashed with a little under 3 gallons and then sparged with two equal sparges of a little over 3 gallons each for a boil volume of around 7 1/2 gallons.

No top off water was needed in either boil kettle or fermentor.

do another batch sparge or increase the size of your first batch sparge. you want more volume going into the pot.

I myself have some trouble with predicting my boil-off evaporation, but I generally go for about 7 gallons or just under that and boil it down to about 5.75 in 60 minutes. 5.5 goes into my 6.5 gallon carboy as a primary fermenter and about .25 gallons is left behind with trub in the kettle.

I did a test today and found on my equipment I boil off a quart every 10 minutes. So for a 90 minute boil I would lose 3 gallons which is just what you lost. I would increase the sparge volume to get enough into the boil kettle. That should help with efficiency.

Thanks for the advice. I'm brand new to all grain brewing. This first batch that I did, I hit my predicted O.G. So what will adding more sparge water get me. What is the difference of just adding it to the brew pot.

Wild Duk - Welcome to AG.
More sparge water, means more sugar going into your boil, higher numbers = higher efficiency. It is not something to worry about as long as your hitting your numbers and your happy with your beer.
More sugar you can extract, less grains you have to buy. It is really important? No. Is it one of the thousand things you think about when homebrewing? Yes.