Display of set current on a variable bench power supply
If I understood you correctly you are designing a bench supply that will work as a voltage source but the current is capped at some user defined value. 

In that case you first set the voltage correctly.
Then you have a very  small resistor which is rated for high current (R1) (search for current sense resistor) and connect it to an operational amplifier (OA1). This ordinary analog signal (A) can now be connected to a comparator (OA2) where one input is connected to a potentiometer and the output of said comparator connected to the switching MOSFET. Providing those circuits with power is easy since you already have +5 and +12VDC. 







simulate this circuit &ndash; Schematic created using CircuitLab

A few comments: 


I could not be bothered to calculate anything but the values given seem quite reasonable (based on my mental model of generic OpAmps)
N-channel MOSFETs tend to be generally better
the amplification of the voltage over R1 is proportional to R3 (and a hard to measure amplifier spec); just calibrate your scale by measuring A with a know current. I feel like printing a scale (after calibration and some calculating)  and gluing it next to the pot is a neat solution
R5 is there to provide some additional hysteresis to OA2, not sure what value and if needed at all
most amplifiers have low input current, you can choose a high potentiometer value
it is good practise to include a fixed resistance in series with the potentiometer so you do not short and burn it
You can display Signal A somehow, for example using an ADC and a display


I hope this was coherent enough; you are also likely to find good application notes when googling for current sense amplifier. It would be nice if people point out errors and such, I will likely have made some.