You're fine. The 50% means 50% of your Total contract, not just 50% of your current points in your account.
One caveat though to keep in mind is that 50% is the cumulative total.
210 points April Use Year, you can bank 100% by Sep 30th.
For the 50% window, you can bank only as many points such that it would not exceed 105 points (50% of 210 points for your example) total in the banked account. Thus if you already had banked more than 105 points, then you couldn't bank any more additional points.
In your example, making the reservation, would not have any previously banked points, so you may bank all 105 points anytime before Dec 31st.
Say for some reason you banked 50 points before Sep 30th. In this case when Dec 31st comes around you can only bank 55 additional points.
Say for some reason you banked 100 points before Sep 30th. Then when Dec 31st came around, you could only bank 5 more points.
Say for some reason you banked 135 points before Sep 30th. Then when Dec 31st came around, you couldn't bank any more points at all.
The same procedure follows later at the 25% window. Keep in mind that MS always rounds down. So for a 210 point contract, 25% is 52.5 points, so your target value that you can bank is 52 points.
It's all really pretty straightforward. Hope this helps.
One final comment. The point total includes your HH contract and any Add-On contracts you may purchase later. Say at a later date you purchase 100 SSR points. Your total is now 310 points, so the magic percentage numbers become 155 points (at 50%) or 77 points (at 25%).
Say you made a reservation and used 175 HH points. At the 50% window you want to bank. You may bank so the cumulative bank does not exceed 155 points. Thus you could bank all 100 of your SSR points and also all 30 of your remaining HH points, since together, they are only 130 points.
Note: The above only applies when the two resorts are an initial purchase and a TRUE add-on, meaning it shares the same base contract number, and both are under the same membership number.
If you ever purchase a resale contract it will often be under a different membership number. In these cases the banking percentages apply to each account separately.
If you have two resorts with different use year months, they WILL be separate membership numbers.
If you purchase an resale contract with the same use year month, it's best to have it put on as an Add-on and not as a separate contract. Disney MAY do this without asking, but it's best to make sure your resale broker works with Disney to make sure it happens.
Besides banking, other factors are involved between two membership number contracts, such as you cannot combine the points to make a single reservation. Say you want 5-days. Then you have to make two separate reservations, perhaps one for 2-days, and the other for 3-days, and then link the two together. However you can never combine points to make a reservation for any given single day. (You can't use one contract for 2-1/2 days and the other for the remaining 2-1/2 days)
When resorts are true add-on's, you may combine points in any way you want to make a reservation. (Subject to the points' home resort 11/7 month reservation windows.)
Well, you got more than you asked for, but I hope it all helps.