Each council is different so it's difficult to identify a "right" way to do things but a general reserves policy, irrespective of the numbers, can be helpful. For most parish/town councils, the JPAG guide suggests that between the equivalent of 3 - 12 months planned expenditure is considered appropriate to hold as general reserves plus amounts held for identified projects being earmarked separately. So, not sure what you mean by "EMR pots had no legal basis".
For my council, the budget is drafted by the RFO based upon a combination of previous year experiences, inflationary amounts etc. for routine expenditure items plus funds for potential projects or one off expenditure items which are usually identified but not necessarily agreed yet. So, for example, our current draft budget for next year includes a notional amount for VE Day celebrations albeit no specific projects/events have yet been identified. Once we have a total expenditure figure, that figure is further adjusted to show where expenditure might come from an EMR amount (in our case, we've been putting money aside for a major capital project) or from general reserves leaving a figure for expenditure to be funded from that year's precept. Clearly this is a bit of a balancing act and not just applying a formula. The final precept figure also takes into account our stated reserves policy which has been agreed separately and reflects the JPAG guidelines.
It is not an exact science to be honest and as you've experienced, an underspend against any project this year can emerge when you get closer to the year end, in which case the funds are transferred to general reserves, possibly to an EMR if they relate to an underspend for a specific project where timings haven't worked out as planned. If appropriate, this may be reflected in a later year's precept calculation if a project doesn't materialise at all.