strike water, why full amount

I'm planning to do a partial mash in the next couple weeks. Why would I want to try and calculate the appropriate amount of water for the whole batch and not just top up as needed once it goes into the fermenter?

Did you read the partial mash sticky above?

You want the right amount of water for the grain. Knowing the water volume and temperature, and grain weight and temp, allows you to predict the mash temp.

You will sparge with another known quantity of water, at a predictable temp. The grain will absorb some, leaving a final amount known. Based in your batch size, you will know how much additional water and extract you need. So, the total water can be calculated, which is good if you are buying or altering your water.

Following the sticky should get you there.