Text in shapes not central

I`m using Captivate 9 and have the following issue:

I want to write a text into the center of a smart shape (i.e. into a rectangle). Moreover I want to prevent Captivate from transforming the text into an image. For that reason I add an empty variable.

Example: If "variable1" is a variable without any value, then I write into the center of the smart shape

This is a text.$$variable1$$

Now the problem: Depending on the browser and the font I used, the text won`t be central anymore when published on my computer. Often it is too far up or too far down.

Does anyone know a solution or some trick to overcome this problem?

Thank you very much in advance!