No you where quite right in doing it proportionally. Every ticket sold increases the chance of the syndicate to win. If you buy 5 shares you should get 5 parts of the win, similarly to the lottery overall.

The charge is fair, you should however make it clear that this is the only cut you take, as its actually rather low. Considering that from the 0.01 you pay 0.0005 per ticket to actually purchase the lottery ticket.

I would NOT run a second syndicate as this would further complicate things. For example taking the big lottery you could simply divide it in shares. A ticket costs 0.25 BTC which is a barrier of entry for some newer players. You with the syndicate are in the perfect spot to enable low rollers to take part in it. Set a fixed ratio of for example 1/10. So each share costs 0.025 BTC + 0.01 in fee's (that might be much as price is so low but would encourage to buy more shares at once so less work for you). Then offer to sell any number of shares. If I would buy 20 shares for example I would actually be paying for two full tickets, its just written differently.

The risk you then have is that in the end you might end up having sold for example 101 shares, enough for 10 tickets but you are lacking 9 shares or 90% of the last ticket price. This is the worst case scenario. Either invalidate the share bought last (you have to be up front about it) or "swallow" the risk and force yourself to buy the last 9 shares for yourself which likely you wont mind as you want to part take anyway (which I think is not a problem at all).

Also you should set a deadline x hours before the draw to ensure you have time to actually purchase all tickets (as you do it manually) and for the tickets to make it into blocks in time.