For a styro, that's about a normal rate of hatch. However, the post-hatch results indicate a bigger issue.
Knowing what kind of leg issues, such as neurological (both legs physically normal but don't work equally), deformed (visible physical deformity), physiological (slipped tendons, red hocks, displaced hips) it's hard to know what is going on with your hatch.
What is your brooder temp at? While chicks and poults do tend to huddle on top of a flock member making distress calls, they should not do that for warmth. A brooder temp of 95-100 for starters will ensure that they spread out comfortably.
I've hatched a couple hundred chicks and poults this year in styro, redwood, and homemade foam incubators and have not had a single leg deformity from shipped eggs, from bought eggs, or from eggs from my flock. While the issues you are facing MAY be caused by incubator deficiencies, they may also be caused by brooding deficiencies, like not enough traction at hatch and during brooding to enable the chicks to get traction. The Brinsea hatch will help determine if it may be an incubation/brooding issue, or a parent flock issue.
When I raise birds for meat or eggs, every bird I bring to maturity or to market reflects a cash value. It's in my best interest to help them survive whether it means flipping them onto their bellies so they can get up, or giving them corrugated paper for the first couple of days so they can easily find food and get traction. For breeders, I do try to select birds that have no issues at hatch, but the cross I am making now is physically unable to hatch without issue. As long as they are on their feet in 24 hours and acting normally it's a success...weeding out will take place in gen 2. I need to build a big enough gen 1 pool to have a gen 2. Gen 2 will be a cross of like-sized birds so hatching should not be so much of an issue.
I do hope the Brinsea provides a better hatch rate, and the brooding success is in your hands. A mother hen will beckon her babies to food and water after a couple of days, once they are active just splashing in the water with your fingertip and tapping the loose feed on the floor paper should get them going. Once one gets it, the rest will follow.