How to be certain a Boardman ADV 8.8 would fit without prior road bike experience? I'm looking to buy my first road bike - an adventure/gravel road bike.

Question: How can I be certain it's okay or not? And should I be looking for a bike that's closer to my estimated frame requirements, and with ideal clearance/reach?

Background information: I went to Halfords today and rode a Boardman ADV 8.8 (size large, 55.5cm) around the store. It's great value for a 10.5kg gravel bike. I'm a 5 ft 11 male with a 34 inch in leg size. So using the livestrong calculator apparently best for me is 57.86 frame size &amp; 76.26 saddle height. 

I loved the bike but i wasn't sure whether it was too small - specifically, i'm referring to the efficiency/comfort of my leg rotation/angles on the pedals. It felt like my legs were coming too high up and meeting some resistance from myself. So the staff raised the seat more and it became noticeably easier. 

The staff didn't want to keep raising the seat saying the bending down could become uncomfortable. Whilst with 1 1/2 / 2 inches of standing clearance between the bar and the crotch, they worried an XL would be too high and I wouldn't have enough reach.

I'm not sure whether it just feels new and different compared to conventional cheap hybrid bikes i've been used to, or whether there's a problem. It is obviously smaller than my estimated frame requirement, but apparently the XL wouldn't work (reach &amp; clearance).

So the question: How can I be certain it's okay or not? And should I be looking for a bike that's closer to my estimated frame requirements, and with ideal clearance/reach?

Thanks for any advice.
This is why people recommend not going to Halfords. (For the benefit of people outside the UK, Halfords is primarily a car accessory shop, which also sells bikes).

If you go to a proper bike shop, the staff will insist on fitting the bike to your properly, rather than coming up with excuses not to. They'll also let you take the bike on a proper test ride, rather than just riding around the store.