Animal companion CR for calculating abilities

[Umbranus]

I am trying to build an aasimar mounted archer with the celestial servant feat that gives my mount the celestial template.
There it states:

celestial template wrote:

A celestial creature's CR increases by +1 only if the base creature has 5 or more HD. (...) SR gains spell resistance equal to new CR +5

It is a huge cat level 10 (9HD) because I went into mammoth rider.

So how do I calculate the SR my mount will have?

[Umbranus]

No idea?

[james maissen]

Umbranus wrote:

No idea?

Ask your DM, but you could suggest that it uses your mount level (10) as the CR instead. This will be higher than it would be otherwise, but doesn't seem wrong as SR is a double edged sword.

-James