Its just so you have some resolution to play with. There is no "magic" to having 50's in the middle.
In traditional mode the master fuel value is how long the injectors open for at 100kpa if the fuel value is 100. Any value in the 100kpa line is basically what percentage of the master value is going to be used.
master fuel trim is bascially just there to let to put decimal points into the master fuel number. ie you cant enter 10.5ms, but you can enter 10ms, and then 5% fuel trim. So 10ms and 10% fuel trim is the same as just entering 11ms and 0% fuel trim.
If you're tuning from scratch I dont see a reason you'd use it. More likely it would be if you put in new injectors that were a few % off your old ones, or new engine parts that changed your VE just a fraction but equally across the whole fuel table
Yes, the RPM range you have set is pretty normal for an RB25. To prevent it switching on/off at cruise just change that TP condition to something that is not in your normal cruise range. I would go to like TP >1% or similar.
