There is little value to this type of a "plan". I use quotes because it is NOT a plan at all - it is only a hope. As a wise mentor once told me Hope is not a plan. This is a TOP-DOWN view of the potential of your business.
The little value is just to give you and the investor a hypothetical view of the potential. So you can give a very large Y ($B) and a small X (<5%) say that there is a huge potential. Remember that if you show something like 20+% in 3 years, then your investor will ask you to hand over whatever you have been smoking :) and you will lose credibility. So... this X% of Y$ market is not useful and if this is all you have put in front of your investor - you will be shown the door.
To make your plan credible you need a BOTTOMS-UP PLAN - a real plan.
A real plan is not just numbers starting from zero customers/revenue growing to some realistic number over 5 years, but one that is supported by ACTIONS that your team will undertake to achieve those. These actions can be in the Development, Marketing and most importantly Sales areas.
Example: If I have an Enterprise business, and I show say 5 customers in year 1, then I need to show actions that may be something like:
- Attend 3 conferences, collect 200 leads
- Target and qualify 50 of them
- Pursue and close 5 of them with 2 sales personnel
These actions will be supported by a hiring (Sales/marketing) and investment plan (for marketing, lead-gen, promotions etc). Now the 5 customers (and their revenue) becomes a believable number.
Repeat this for Years 2-5 and you have a real plan supported by actions that shows the investor that you have thought through the details. By Year 5 you may have 100 customers at ~$500K a piece which will put you at $50M revenue. If your addressable market was $1B then you have reached 5% of the addressable market.
Hope this helps.