I believe you should be fine as you have the letter from your employer as supporting document. I can understand your worries coz me too was travelling abroad for business 60-70 days in 2007 out of 90 days.
The savest bet is to bring more supporting documents from various sources although they asked for at least 3 only. Even invoices for your gas bill is as good. At the Home Office, after you surrender your file to Counter 1, the chap will select a few relevants ones and return you the balance.
Hope this help.
C
