Sunbeds: to build a tan, should I increase the duration or intensity?

First of all before you all start lecturing me on the dangers of using a sunbed (of which I am aware), I should let you know that I don't normally use them. In fact I rarely sunbathe, (approximately 1-2 weeks a year and even then I don't go out in the sun without a minimum of factor 20 SPF lotion) and throughout the year I wear moisturiser with an SPF of 15 (for winter). However I am going to Thailand in a week and I am sensitive to the sun (I get a sun rash very easily), so for this reason I am taking anti-histamines and also beta carotene tablets and omega 3 oil capsules to help protect my skin. A leading dermatologist also recommended that I use a sunbed a few times a week and very slowly and gradually build a base tan so that my skin isn't "overwhelmed" when I get to Thailand. I'm not skin type 1 or anything, but my skin rarely sees the sun. I've been using a very light bed at the tanning salon. I started with 8 minutes and have built up to 12 over the course of a few weeks. My skin has been fine with this and I'm using loads of moisturiser afterwards. I think I need to increase the intensity now (a week before my holiday) because my (very) light tan is fading.. :-s Should I increase the intensity of the bed I use or the duration of my session? The salon has vertical beds but I've heard they are strong.