Shouldn't be very critical; you could do a clip test to determine a fixing time, or just go with something like 5 minutes at 35-38C and call it good (for 1+4 dilution). Fixing will be fast at C41 temperatures.
If you have a pH meter or strips, you could even bring your acid rapid fixer up to pH 6.5 and have basically the same thing as 'proper' C41 fixer. Use ammonia to increase pH.
Shouldn't be very critical; you could do a clip test to determine a fixing time, or just go with something like 5 minutes at 35-38C and call it good (for 1+4 dilution). Fixing will be fast at C41 temperatures.
If you have a pH meter or strips, you could even bring your acid rapid fixer up to pH 6.5 and have basically the same thing as 'proper' C41 fixer. Use ammonia to increase pH.