Use a pool the gives you the ability to create subaccounts, most importantly the pool should let you pick different wallets for different subgroups, a good example of a pool that does exactly that would be viabtc pool.
The 75 TH is for example a buffer in the hashrate of A, B, C or D drops below there fixed number.
This can't be done on a pool level alone, not on a local level alone either, you will need a lot of work to get this done, here is an idea off the top of my head.
You create 5 subaccounts A, B, C, D, and E, E will have the buffer hashrate, in the pool settings set a "low hashrate notification", this will send you an email once one of the main 4 accounts fall below the threshold you specify.
up to this point, everything can be done by you or any other person, the following will need a programmer to write a script which would intercept the email and then access the E miners on your local LAN and change the worked from Worker.E to worker.B.
Those 4 lines are easy to write and read but certainly are not easy to make, and I don't see how applicable it is, if I was you I would "force" my clients or whoever owns 4 accounts to accept my way of settling hashrate drops.
Say they get weekly payouts, every Saturday night you check the average weekly hashrate, account A has 100TH average > his credit = 0, account B average is 40th and he must get 50th so he has a credit of 10th.
Say he got 5 BTC for 40th, it means his balance is 1 BTC, now with this scenario, you must disable auto-withdrawal for account E (the buffer account) so that it always has a buffer balance and you don't have to send BTC from E's pool to E's wallet to B's wallet, you can easily manually withdraw 1 BTC from E's pool account to B's wallet and you are done for the week.
To sum up, if the hashrate you are trying to manage is small, aka the TH or the below 5-10ph then it isn't worth the efforts, to settle payments daily or hire someone to write a script for you and charge you a fortune.
