Welcome Bobbydazzler,

This topic has been raised many times on this forum and opinions are varied on the best staking plan to use from level stakes, percentage of bank, Retirement plan to loss chasing schemes. My preference is 2% of highest point of bank (non-reducing). If you want to be a little more conservative you could use 0.5% or 1% of bank.

Check this out on your 18 months results to see how it suits your 2 systems.