'directly determine' is a correct and usable phrase in written English.
You can use it to indicate that something is a primary factor in determining an outcome. For example, "The integrity of the voting process will directly determine the legitimacy of the election results."
The Harvard VCM employs an experimental determination of NP agglomerate volumes to directly determine effective particle density.
These gambles can be tremendously lucrative, but they don't directly determine the real (or "spot") price of oil.
But if the EU's elected body could not directly determine policy during the biggest crisis in the union's history, some might ask, what is the point of it?
This means that they cannot directly determine whether a particular blue straggler star originated in a binary system whose components coalesced or is the wreckage of two unrelated stars that happened to collide.
"How the UK decides to respond to these developments will directly determine the level of security available against the threats we face," said Parker, who became director general of the counter-intelligence and security agency six months ago.
They couldn't know that having the teams play 14 of their 17 games in the last 10 weeks of the season, including 7 in the final 10 days, could directly determine the outcome of the race.
Chromatographic analysis is an effective strategy to directly determine drugs.
Job evaluation schemes do not directly determine rates of pay.
A carefully selected order will directly determine the robustness of this feature and the running speed.
This is quite an easy problem and one can directly determine that (theta) equals (alpha).
The Mössbauer spectroscopy method is currently the best method to directly determine the sulphur-iron compounds.
