Maybe the trick with the ERC series is to go around the paddock at the next meeting having a quick look at the tyres being used and make a list of all of them. Then make that the list of acceptable tyres for the series (assuming you don't find anything unacceptable). Then anyone wanting to run anything different can apply to have another tyre added to the list - a couple of minutes on the internet should show whether it is in the spirit of the series and if it should be accepted. That way no-one turns up saying 'but it is a WOF-able tyre so I should be able to run it'. That conversation has been had well upstream of purchasing. Set a criteria e.g. min 4mm tread covering 3/4 of the tyre or whatever is desirable so there is a good basis upon which to reject or admit new tyres....