Depends if you want your AVC's to benefit from PSE (Pension Salary Exchange)....this is my understanding:
The new pension is still going to work with PSE so if you make a level of AVC contributions that would take your pre-tax
wage (left hand side of the wage slip) below the National LIving Wage/National Minimum Wage then I think your AVC
contributions (or part of) would be taken out of your post-tax wage (right hand side of the wage slip).
If you keep your AVC's at a level that ensure you stay under PSE then you pay (the equivalent of) £72 to get £100 of
contribution into your AVC account. If your level of AVC's take you out of PSE, then you'll be paying (the equivalent of)
£100 for the £100 into your AVC account due to losing the tax relief from PSE.
Any salary sacrifice items you've got from RM Bundle (health insurance etc) that appear on the left hand side of your wage slip will already
take your pre-tax pay down towards the NLW/NMW so you have to be a bit careful working out what your level will be to stay in PSE. HR won't advise you what your limit would be (as it's 'financial advice') so you have to do a bit of working out. In RMDCP To begin with I set a number of £ under but then had to adjust it down a few quid to bring it back under PSE. I eventually set it as a percentage to proportionally increase the amount I paid in AVC's if I did overtime.
Suggest you check out the AVC booklet:
https://rmcollectiveplan.com/documents-forms