your conversion kit comes with a 16T.
Id get a couple more Shimano DX cogs for fun and giggles. You should be able to go up or down 2 teeth with the same size chain. Bring a chain whip and lockring tool in yer camelpack and experiment trailside.
:mrgreen:
DX cogs come in: 14T,15T,16T,18T
Wiggle has gusset brand cogs in 14, 16, 18 and 20t for £3.99.
Despite what anybody says, do not try to break your cassette and use hyperglide cogs--your chain will jump and you will get hurted. Old 80s Uniglide cassette cogs, however, work great.