How low can you go... well all depends upon your exact goals and just how much you want to ensure better health. No one can really tell you the correct answer. Your genetics, current look and diet will dictate the rest. Although I will give me as an example and the same could be applied to you and others. If you have a decent base then any base of hormones will do the job. I could cut now on 300mg test and look great for me but again what are your exact goals. If you want to be as big and lean as possible then 300mg test will obviously not cut it. At the same time no one needs a shitload of steroids to just maintain muscle when losing body fat. Nevertheless, every little thing you add will/should only benefit you.
I could cut now on 3-400mg test and look great for me. However adding masteron will assist things and create a slightly better look. My body loves avar and if I were to add that again a better look... harder and more vascular etc. So it's just about weighing up your exact goals and how much you are willing to push things but you can create a great look without having to abuse yourself. There are guys on here who think you can't look freaky unless you run 3+ grams of compounds including tren, avar, halo etc etc. Whilst taking all those things and at high doses will only help make you look freaky it's not essential. Your cycle is plenty for a fantastic look. Not as a starting cycle and you have to move up but those doses and mixture of compounds are great for someone who wants to look great without abusing themselves. It has everything you need and the doses are not high but they are enough.
When I go up to 60mg avar+ I get bad acid reflux so 40mg suits me perfectly. Proviron I am fine with and more could help create a harder/better/freakier look but 25mg is enough but 50mg is likely better and 75mg even better but quite honestly any dose will help you. The same for masteron yes you could add 1 gram of mast and it would assist a certain look but it's totally not needed and may go against you so 200-600mg mast is gtg and your 350mg sounds perfect being 50mg per day. Guys really over complicate drugs and often as long as you have a decent dose (low, medium or high) of the right compounds you can/will get results . Yes more is usually better and sure for freakiness you need higher doses but your proposed cycle looks great to me. I have actually used it (the exact doses) myself when cutting.
As others have stated add in 5mg cialis daily for the prostate and to assist your look/results. The proviron is not needed with mast in the mix but it wouldn't hurt but sure maybe add it in the 2nd half of your cycle for an extra boost.