Four figures and you have an answer, or the reason it depends. If you do construction work for other firms the answer is not the one you expect, and we say why before any arithmetic. No account and no payment.
You charge your customers VAT as normal, but you hand HMRC a fixed percentage of your turnover VAT included, not of the figure before VAT. In exchange you stop claiming back the VAT on what you buy, apart from capital goods over £2,000.
On construction work done for other VAT registered firms and reported through CIS, you no longer put VAT on the invoice. HMRC leaves those sales out of the flat rate scheme entirely. So on that part of your work you gain nothing from the scheme, and you still lose the right to claim VAT back on materials.
If in a quarter you spend on goods less than 2% of turnover, or less than £250, HMRC puts you on 16.5% whatever your trade is. Goods means goods: not tools, laptops or phones, which are capital spending, not fuel, not food and nothing that is a service. That is why plenty of people who sell only their labour end up on 16.5% rather than 14.5%.
It assumes everything you sell is at the standard 20% rate. It does not cover capital goods over £2,000, cash accounting, the first year discount, or the rules for leaving and rejoining the scheme. These are your figures from last year, not a promise about next year. Before you change scheme, ask an accountant.