There is a relationship between body fat levels and the ability to gain/retain muscle.
Generally speaking (in my personal opinion), a bf above 12-15% will not do anything significant in increasing your ability to gain muscle, if you are a pure powerlifter with no aesthetic/health requirements then being higher than this may be worth the 1-2% strength gain.
Going below say 10% you will find it progressively harder to gain muscle
Going even further say below 6% you may find it hard to retain your existing muscle and going lower will may cause muscle and strength loss, this is quite unhealthy.
This is why you won't find BBers with a BF of 0%.
Taking the same individual but with BFs of 12 and 5% for example, the guy at 12% will have a higher TDEE than the guy at 5%. This is partly due to the body being more economical with energy, which in turn reduces recovery ability. It is also more energetically preferable for the body to loose muscle as muscle burns energy even when inactive.
A similar effect can be caused by eating way under you TDEE, which is why gradual dieting, not crash dieting is not recommended.
Scooby’s target audience is the overweight individual; hence his advice is true for the most part. However his advice does not suit the skinny teen who doesn’t understand why he can’t get bigger yet refuses to lose his six pack temporarily.
Point being, bulking/cutting may work better in some situations.
Also Scooby’s calculator is only an estimate, true progress can be measures by taking a workout log tracking strength week on week and by tracking weight loss over a period of time (your bodyweight will fluctuate hourly by a few lbs making week on week measurements meaningless).
Hope that's cleared a few questions
