Make sure that you don't file any loss (reducded tax liability) for the tax year you are going to use for the 18.6, after she has got her visa to the UK. UKVI will check with HMRC for that when she applies for future visas.
Your acountant might suggest that you do that but you must follow the immigration rules and doing that would be classed as decpetion (to get a visa) if it means that you didn't earn at least 18.6 of taxable income that year.
