Don't go with investors asking. They will stand to gain as it dilutes yours.
Use a simple calculation:
1. How long is it going to be the until the next round? Typically it is 12 months to 18 months
2. How many employees are you planning to hire? Now for each position, how much are you planning to allocate? This is what you need.
If you come up with the right number with the backup data, you should be ok. You can always increase it if you need more. What if you need only 10%, and in this case, the post-money per-share value will have the unnecessary 10% unused shares in it. Without the right number that you can come up, you may be forced to go with what the investors are asking for. So, do your math.