usage of 'before'

Examples are quoted from English Grammar Usage.

Expression +time+ago
I met her six weeks ago.
I met her a long time ago.

An expression with ago refers to a finished time, and is normally used with a past tense, not a present perfect tense.
However, a present perfect tense is used with since ….ago(as with since+any other time expression)
We have been in Bangalore since about ten years ago.

Ago is used with a past tense and a time expression to count back from the present; to say how long before the present something happened. Before is used  in the same way(with a past perfect tense) to count back  from a past  moment.

I met that girl in Bangalore three years ago.( not three years before or before three years)
When we got talking, I found out that I had been at school with her husband ten years before.(Not ten years ago.)

We can also use “before” with a present or past perfect tense and no time expression, to mean at any before now/then.

Have you seen that movie before? Have you been here before “ Yes, I was here a year ago.

However, a present perfect tense is used with since ….ago(as with since+any other time expression)
We have been in Bangalore since about ten years ago.

Sorry, that is not correct.  You can't use 'ago' with present perfect + since.  Yes, time expressions with 'ago' do identify a point in time when something happened or when something began using simple past, but grammatically they cannot identify how long something has continued up to now with present perfect. So it is a grammar mistake to use 'ago' with present perfect + since.  Here are some correct options:

We have been in Bangalore for about ten years.

We have been in Bangalore since about 2002.

We moved to Bangalore about ten years ago.

We started living in Bangalore about ten years ago. [unnatural but grammatically correct]

I arrived here two months ago.