Hi,
to improve precision i recommend to use a mosfet instead of BJT,
With a MOSFET there is about no gate current. But with BJT you have an bas current.
The Feedback is with R49. R49 sees load current + base current. So base current causes error.
Additionally the R1 causes current through R49 giving additional errors.
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To improve stability, i´d replace the R1 with a c (best a foil c). An additional resistor between this C and R49 may further improve stability.
The OPAMP needs to be unity gain stable.
Klaus
PS. look for 4..20mA constant current circuits.