How to customise the speech controller in text to speech feature?

In **System Preferences** > **Accessibility** > **Spoken Content**, I can set a shortcut so that the computer can speak highlighted text. Further, I can change the speed to how fast I want by opening the following and changing it there

open /Users/**/Library/Preferences/com.apple.speech.voice.prefs.plist

I set the voice rate to 800 which is about 700 words per minute. The controller gives the option to speed it down by 0.75 or speed it up by 1.25 I'm wondering if there a way to change those to 0.85 and 1.15 for example? I'm talking about the turtle buttons [in the black controller](https://support.apple.com/en-gb/guide/mac-help/mh27448/mac) .