Honestly, I don't think you will be able to control the boost down that low with only one Tial 38mm and here's why...
The general rule of thumb is, if you want to run low boost levels, you need to run the largest wastegate that you can in order to bleed off the excess exhaust flow. Now, with your motor being a 4.1L, even though you only have a small 61mm turbo, the exhaust flow of the 4.1 will require a larger wastegate. You could try the smallest spring that they offer, but I'm afraid that it would only lead to slow spool up (caused by the wastegate opening up prematurely due to a soft spring) and boost creep issues. A Tial 44mm wastegate should be enough to allow you to run the boost down to 10psi. The other choice you have would be to install a second 38mm wastegate onto the crossover pipe with a matching low pressure spring in it. It's a tough call tho. One question, why are you trying to run the boost so low? You should be able to run at least 12-14psi of boost on pump gas, unless your motors CR is higher than 9.5:1. The rule of thumb for running high boost is you run as small a wastegate as you can get away with.
HTH
Patrick