Hello, dear,
you 45 ° is taken from model, and this size is correct (but you see this 45 ° as 30 ° due to isometric distortion).
Generally, there are two ways to create isometric view. They are based on:
1. 3-d view.
2. Section view.
Each has some its for and against.
If you want to keep your actual 45 ° in 3-d view, you can write any value you want into your angular dimension (for example, 30 °). Read this.
I usually use isometric based on section views.
Open floor plan, create 2 sections as pictured here:

Open section 1, rotate section 2 to some angle. For 30 °, rotate in exactly for 35.265°.

This will make your isometric exactly 30 °.

Now some for and against:
if you use 3-d view: you can explode view — its much easier to create explode schemes.
if you use section view: you have much richer tools for annotating the view (at leas you can use annotation families and draw detail lines! Strange, it is impossible in 3-d's).
!!! If you take any size from isometric section view, be sure to take into account the correction factor of 0,817.
I place this picture on my drawings:


K = 5307 / 6500 = 0,81646...
If you anyway need 6500 instead of 5307 in your section view, so override the dimension text...