Micro/minor g adjust optimisation

• Avoid making a variable saved in g_adjust

• Slightly optimise g_adjust by pulling repeat calculations out of inner loop

Local testing shows this to be 10-20% faster than next when compiled with gnu-5 using -O3. It seems next can approach this performance if compiled with -march=native -Ofast. This suggests that these changes are perhaps doing what the compiler would like to do but can’t at O3 as it would reorder the floating point calculations.