Oh, the eternal budget dance with Google Ads. Love that you're thinking about it - most people just throw money at the screen and pray. Here's how I handle it without losing my mind (or my client's savings).
Work backwards from your break-even cost per action. Take the value of one action - say a paid consult - and multiply it by your conversion rate. So if one in ten paid consults actually signs on, that consult is worth about 170 quid to you. Then decide how many conversions you want per month - 30 is a solid target. So 170 x 30 = around five grand. That's your absolute floor for Google spend, management fee not included.
You can reverse-engineer it further up the funnel too. If one in ten website visitors becomes a paid consult, each visitor is worth 17 quid. Suddenly your cost-per-click target makes a lot more sense.
Just don't forget that the platform loves to eat budgets whole if you don't keep a leash on it. Hope that helps - and if it doesn't, well, at least you've got a number to argue with your boss about.