Unfortunately I don't know of any simple ways. Like you mentioned I just do monthly projections and quarterly summaries. You just have to know what's an input and what's a result. With my business I have to go the other way, from how much I expect to hire to how much revenue that creates. There's not much more to it but you'll probably have a few more dependencies.
If there is a process, I would put it something like this - none of these are concrete facts so it's just a starting point for discussion:
- Map out the causes (time/money invested) and effects (resources needed) in your business process
- Plan what you expect to put in at the start, and the range of outcomes you could see at each following step
- Identify the potential hiring needs and urgency for each department so you know what you're looking for and the priorities
- Try to get more applicants than you expect to need, and keep them interested until you're ready to hire them
- Identify signs that tell you to update the plans
Now to explain that: Like Apollo says, you need to be flexible. If you can, use contract/part-time work to get extra help and get to know people before you need them full-time (but find people who are actually good and have long-term potential so you can make them full-time later). Not being able to give firm commitments to everyone makes things more difficult but it also lets you prepare a lot further ahead.
Are the sales projections proven? Not just in the total revenue/customers but in when it will happen? If you're absolutely sure how much business you're going to get in the next quarter that's one thing, but if there's a range make sure you understand it and prepare for the full range of possibilities.
As a planning exercise, there are two things that stand out here:
- Each department may be different. For example if your support can't keep up to the new customers you could quickly lose them, but if development lags behind a bit you can probably catch up later with little disruption. On the other hand sales may be an area you only grow when you have the extra cash to invest in hiring salespeople.
- It sounds like this will be interconnected with most or all of your top priorities this year; reviewing the hiring plan quarterly may not be enough. Not that you need to do it every day either, but I would want to watch closely and if things have changed sufficiently, re-examine the assumptions and plans. When you're growing rapidly, planning doesn't have to happen on a fixed schedule.