Your cash benefit is calculated by multiplying your total spend on R&D by the refund rate that applies to your company.
If your company is paying tax then the refund rate is 13.5%.
If you company has zero revenue then the refund rate is 43.5%.
If your company has some revenue but is not paying tax then the refund rate is somewhere between 13.5% and up to 43.5%. The actual rate depends on the size of your company's losses, the more losses the closer the net benefit refund rate will be to 43.5%.
The main thing to remember is that you can get a cash refund without paying any tax.