Farah,
I do not think that is correct. You can stay out of the country for more than two months in year. In fact, you can spend 90 days in a stretch. You shouldnt be too worried about that. When you fill in your application form, you will have to note down all your dates of departures and arrivals for the past 5 years. I know that the immigration does not stamp your passport while departing from the UK. However, you should have the arrival stamps in a different country. So you need to figure out using all the stamps that appear in your passport and make a list. And yes, a letter from the employer is always a good thing to have in case if they want to see it. Good Luck with your application.
Amir