HOB overflow, sump and flow rate question

[BeardDadtheDILF]

Hello everyone, I am still in the process of setting up my first tank and have more information in my build thread, but at the moment I am looking for a direct conversation around an overflow and my sump. I also would like advice on what I may be overlooking or need to prepare for.

The stores around my town finally have stock of 75G but unfortunately all the tanks I found have tempered sides so I will be going with a HOB overflow. I am planning on purchasing a PF-1000 from Eshopps, I also have a R-200 sump from Eshopps. Both are rated for 125G - 150G. I went with the R-200 because of the skimmer compartment and the extra refugium size the R-200 has two bulkhead inputs and the PF-1000 is when the Eshopps line starts two outputs. My main questions are:
Will this be to much flow?
How do I determine flow?
What do I do about backflow(?) if something should fail?

I also have not started my search for a return pump. Will I need to determine flow prior to getting a return pump? Can I simply put everything together and get valves for the return side?

One last question, has anybody had any experience with using the pipes that come with Eshopps products good/bad?

Thank you for reading and for any advice.

 

[Retro Reefer]

The amount flow is dictated by your return pump.. The pf-1000 is rated at up to 1000 gph, for these to operate safely and effectively they need to be pushed to their upper limits which may be to much for the 70.. you could remove one of the U tubes to slow down the over flow if needed. The amount of back flow into your sump if something fails will be determined by how low in the water your overflow box and return line are.