Keep in mind there are several important differences between squatting and using this type of leg press machine:
In addition to the weight plates on the leg press you are lifting the carriage holding them and in addition to the barbell during the squat you are lifting most of your bodyweight.
Most leg press machines load you through the pelvis rather than the spine - the barbell squat requires a significant amount of back strength while most leg press machines require none.
Depending on your foot positioning and range of motion the average levers the hip and knee extensors work against will vary to some degree between the two.
On older or poorly maintained leg press machines friction is also a factor.
Leg presses with the same track angle but different seat angles can vary in difficulty due to the starting angle of the hips - the more reclined the seat the easier the exercise, all else being equal.
Due to the additional risks involved in squatting if you are not sure how much you can squat safely you should not guess based on other exercises. Start with a light weight, test for a few reps, add weight and repeat if necessary. Once you have started to squat down with a barbell on your back is not the time to find out you overestimated.