Sumit has already given an answer to this on the other channel, ill add my 2 cents on top of that:
1. Before everything decide on the org philosophy of what range of pay masters you want to be? Are you going to in the p100 or p50 or p25? That will define this % of Revenue metric to a large extent.
2. This would be recruitment, rewards, payroll, etc.. there are companies which may not have separate skill towers but they will have broad teams - COE, HRBPs, Shared Services.. for budget allocation, it mainly depends on the headcount and headcount depends on the overall company headcount and priorities... additional budgets are provided for tech integration, L&D, admin
3. Some Orgs have defined a HRBP:People ratio, but I would not index too much on that. Go for a span depending on the capabilities of your hires and what are the business priorities and work to be done.
4. Rule of thumb: Pyramid.. but it will be different for different functions.. for e.g. sales will be a pyramid with more people at junior level and less people at senior.. R&D may be a square instead of a pyramid.. Strategy might be an inverted pyramid
5. Google