Lockup equipped 727, 42/44/46/47/48 are not modulated lockup designs.
You will never get zero slippage as long as you have power coming from the engine and vehicle “resistance”.

When you get too much clutch slippage in lockup you will set a code. 60-100 rom is quite common.

If it takes too long to achieve lockup you’ll set a code too. Once the lockup solenoid is activated a timer starts and it must achieve a certain amount of lockup in a certain amount of time. If you don’t that’s when the code sets.

All part of OBD,