I agree with @Mystify, @jrowe47, @unsynchronized.
I highly suggest that you spend several weeks or a month consuming just the regular Soylent amount. See how it affects your body first. After all you are making a huge change to your diet, you need to get a baseline before you start tweaking things.
Also I would suggest against decreasing the amount of Soylent that you intake just to lose weight. In a normal diet it makes a little sense to eat less to try and reduce your calories. In a normal diet you are getting a lot of excess stuff, just so that you can get the required nutrients that your body needs. This is the exact opposite of Soylent. Soylent already contains an optimized amount of micro and macro nutrients for the average person. So unless you are quite a bit smaller or quite a bit bigger than the average person you should be just fine with suggested Soylent amounts.
If you dig through my old posts, you will find a formula for either losing or gaining weight. It does not take into account different metabolic rates, but it should give you a pretty good idea of the amount of calories you should be consuming. You just have to know what your desired weight, and desired body fat percentage should be.
One last warning. Be careful taking extra supplementation while on Soylent. Meaning that if you do decide to consume less Soylent, be careful with what vitamin you decide to take to make up for the lack of micro nutrients. Based on the amount of calories that is required in my DIY version, I was unable to find a multi vitamin that contained low enough amounts of some very toxic nutrients. This forced me to start making my own multi vitamin that I take. If you think that consuming 2/3 of Soylent and then taking a Centrum to make up the difference, you might be mistaken. I have not run the numbers, but I would be really concerned that you will reach toxic levels of certain nutrients.