Refrigerant pressure in an idle unit? (compressor not running)

[Chris3000]

Hello,

I am new to this forum so sorry for any mistakes

I'm now designing my first refrigeration unit and I have some problems, could you please check my questions? (Also, I checked your forum, went through lots of Google browsing + books about refrigeration but no luck for anwsers).

I am now designing a reversible heat-pump with air and water heat exchangers. It has a Carel E2V EEV and a Danfoss Saginomya 4-way reversing valve. For refrigerant we chose R410a. Evaporating is at 7,5 C (+5K), condensing at 45 C (+2K subcooling). We already completed a prototype but I'm holding back to run it for the first time since I want to clear questions below.

My questions are: I've learend as a rule of thumb, a way to estimate refrigerant charge. I used a formula: 1/2 of evaporator volume + 1/2 of condenser volume + volume of liquid lines (neglible in my project) + 20% safety factor. The density of R410a in the liquid state is around 940 kg/m3. I calculated that I need to charge with ~540g of refrigerant. Is this method any good?
Could you share some other methods of calculating refrigerant charge?

Other question: after charging the unit with 540 grams of R410a, the pressure equalized at around 6 bar (external temperature is about +15 C). I've set in the electronic control the cut off pressure on the suction side to be at around 8 bar (0,0 C for R410a) to prevent frost accumulation or freezing water.
So the problem is: will the low side (suction) pressure rise to the 9,94 bars (for 7,2 C evaporationg temp) from 6 bars in idle?

My question is:
Is it normal for refrigeration units to have lower pressure in whole system when they compressor is not working, compared to the suction pressure when compressor is running?

Thanks for any input, all and any help will be appreciated !