Here are some steps you can follow to quickly mark all entries that fall under a specific Category as Tax-Deducibtle:

1. Be sure you're in the Entry section by clicking the button marked "Entry", down at the bottom.

2. Click the magnifying glass symbol inside the white Search field up top and select the Category Search Option.

3. Type the Category designation into the Search field.

4. Click once over any entry and then go up to click the Edit menu at the top of the screen and select the Select All menu option.

5. Go up to click the Entry menu up top, slide down to hover over "Mark", and then click the As Tax-Deductible menu option.

Unfortunately, there isn't a way to have entries you enter later receive Tax-Deductible status automatically when a particular Category is used. But, perhaps you could begin using Categories that include “Tax-deductible” in them. For example, where you might usually use “Medical” in the Category field, you could put in “Tax-Deductible:Medical” and then you’ll be able to filter for “Tax-Deductible” later when you need to see just those items.