To break down the 1:1:1 method. 1 part starter, 1 part water, 1 part flour.
Assuming after I discard half and left with 75g starter, would I then add back 25g of starter from the discard then 25g of water and 25g of flour?
No, your new starter IS COMPRISED ONLY of the 1:1:1 (ie. 25g old starter plus 25g flour plus 25g water) for a total weight of 75g. If you are always using the same jar, you need to know the weight of the jar in order to determine how much residual "old starter" is left when you are discarding.
I manage my starter differently by using 2 jars. When it's time for a refreshment I remove 25g of "old" starter from Jar A and combine this with 25g flour and 25g water. This new mixture, weighing a TOTAL of 75g, goes into Jar B. This is your "new" starter. Jar A is cleaned and washed.
By always transferring your starter into a clean container, it's easier to gauge changes in volume. Not only this, but a jar with clean sides is less likely to attract undesirable molds that can sometimes develop in a jar that is never cleaned out.