I had a look at Folic acid in Wikipedia, and it has a useful table. 400 micrograms id the RDI (recommended daily intake) for an adult & 800 is the safe UL (upper limit). For a preg woman the figures are 800 (rdi) & 1000 (ul).
Pregnacare preconception has 400 micrograms of folic acid, i also have separate fa pills which are also 400. I think i will take both, giving me 800 a day, especially as i am worried about what caused my mc.