Unfortunately the % is a mathematical calculation based on the votes received, so there is not a way to force it to add up to a particular number. If more than one item is rounded up you will see a total that is greater than 100% and if multiple items are rounded down you might see 99%.
Here are two examples:
If you have 30 voters and 10 vote option 1, 10 vote option 2 and 10 vote option 3, the result would be be 33.33 for each answer. These would all round down to 33%, so your total on the slide is 99%.
If you have 30 voters and 2 vote for option 1, 14 vote for option 2 and 14 vote for option 3, the results would be 6.67%, 46.67%, 46.67%. These results would be rounded up, so the results on the slide will be 7%, 47% and 47% and the total is 101%.