It also depends if your tippers are buying coins in the Tapas store or if they're earning them through watching ads.
A bigger cut is taken from bought coins because of the Apple fee that Tapastic has no control over. It breaks down 30% to Apple + 10-15% to Tapas in fees for bought coins.
For earned coins, there's only the Tapas cut of 10-15% taken out.
Try to get the word out to your readers that to truly get the most supportive bang for their buck it's preferable that they earn coins through watching ads and it's free for them. Win/Win
Or if they want to spend money on you, recommend donating through patreon/paypal/or other donation site where you'll get the full amount of that donation.