I`d simply shave the top off and be done with it...as one of the posters above said....it`ll continue thinning anyway and you`d be making `work`for yourself trying to sit a system in behind your front hair ... it could look `off` because system-fronts are graduated in terms of density, unless you ask for non-graduated, and you could have problems either way getting that front bit of your own hair looking `right`with a system sitting behind it...any slight differences in colour/density will catch the eye---the front hairline is what the casual observer sees first....
Your hair seems to have a natural centre-part too .... just order that also....the crown-point has an obvious `swirl`or change of direction...send in those pics so they can see it and they`ll tie the hair into the lace so that it copies that. As Reubin suggested, go for a low `starter` density...step it up on subsequent orders..... I`d even go lower than 55% --Toplace densitites are relatively `high` in terms of what you might expect.... I`m sure John can easily advise on what will work best for you...so don`t sweat over getting density figures bang-on....just send in pics and your order can be adjusted to suit by John.Make your template and mark `front`and `back`on it so there`s no confusion at the factory. Tek a few random hair-samples from around your head..remember...hair at the back/crown tends to be darker than front-hair so don`t take it all from the one spot [i.e. crown]...you`ll need a sample approx. the diameter of a pencil and about an inch long...tie/hold it together with a small elastic band. Excuse the shaky line I`ve drawn...using a touch-pad on a laptop so it`s not too steady, but I`d cover that kind of an area....sit the front edge of the system on your natural hairline.... it wil look off if you go too far away from that [forward or backward] ..the general wisdom is to replace what you`ve lost.... I can`t see your sides, so I can`t say anything about your temple-area, but again, just fill in any `gaps`which weren`t there before. Go for Swiss lace for the moment.... I see you have the hair swept back in one pic..you can do that easily enough with Swiss as it`s invisible to all intents.