With the letter "NSF" is usually meant "dragload" (the load accumulated from negative skin friction, n.s.f.). If a pile has a dragload of 20 tons plus a dead load on the pile head of 15 tons, then, the positive direction forces must amount to that same total amount of 35 tons and the pile will have a bearing capacity of a minimum twice this, that is, 70 tons. The factor of safety is the 70 tons divided by the dead load of 15 tons, that is, 4.7. (Now, where does the 35-ton value come from to which the pile was to be driven?) If you accept this factor of safety as indicating a completed and conservative design, perhaps you would be right, and perhaps you would not. It all depends on what the settlement is at the point of equilibrium, the neutral plane, where the maximum load of 35 tons occurs. If the soil settles here, then the pile will settle an equal amount and the design is in trouble with the serviceability.
If you have difficulty following my reasoning, go to the search function and search the Discussion Forum for "dragload". You will see a large amount of previous entries on the same question and I suggest you read them.
I am aware of the I used the term "bearing capacity" in my reply, which may appear to conflict with my other reply this evening. To avoid implying a conflict between the replies, for a pile, bearing capacity is defined not really by a formula but by the maximum shear along the pile shaft (shaft resistance has an ultimate value; a "shaft capacity" does exist) plus the resistance at the pile toe occurring at an enforced toe movement of about 5mm to 10mm, as a toe capacity does not exist.
Oops, I forgot to comment on the Hiley part of your question: Please, note that the Hiley formula (and all other so-called dynamic formulae) is a relic of the past that has no place in modern engineering practice. You'd be better off hiring a local soothsayer to tell you the capacity from looking at the innards of a chicken. If the soothsayer has experience of piling and construction, his value is probably closer to the true one than that you get from a dynamic formula. If he has not, at least you'd get a dinner out of it. Instead of the formula, you should use the wave equation, GRLWEAP, and you will get a correct representation of the actual conditions of your site, hammer, pile, etc. You will still be relying on input selection, that is, assumed values, so you need to calibrate your input assumptions with measurements using the Pile Driving Analyzer with CAPWAP analysis. Don't do the 'calibration' for just one blow, through. Do it for blows at different observed penetration resistances (mm penetration per blow) and, preferably, for different settings of the hammer (if you can vary the hammer input). Now, you are getting somewhere, but you are not done. Next step is to correlate the PDA analysis results to the soil profile and a static analysis (effective stress analysis, not total stress, it leads nowhere). Finally, evaluate the influence of remolding during initial driving, set-up after driving -- you should consider using the PDA for both initial driving and restrike. Agreed, this takes a bit of effort -- not much though -- and needs the services of an experienced geotechnical engineer. So what! Compare the case of your consulting a physician because you have a problem somewhere in the central region of your body. Would you be happy with his pecking on your chest bone, twisting your left ear counter clockwise, mumbling a string of words, and blowing smoke out through the right nostril, and declaring that you will be ok if you walk backwards three times around your house next moon-free Friday night? Of course not. Well, that physical examination is akin to the 'quality' of 'engineering' based on a dynamic formula plus a borehole log with a few SPT N-values . Please join in with engineering standards of today.
Sorry for pouncing on you. I am probably most unfair to you and I apologize. However, your harmless reference to the Hiley formula opened me up to my disgust of the many, far too many, 'engineers' who make far-reaching decisions involving large sums of tax payers money and the livelihood of hard working contractors just because they could not spend a few hours of their time to apply proper techniques in their design work. As to their taking courses to keep up with things -- heavens!