Be very wary of cheap Chinese C trumpets, I got one for a friend, the open notes were all flat, we fixed this by shortening the tuning slide by 1/2" the maximum without moving the forward brace. We then found the valve slides were all wrong, the 1st and third were midway between the Bb and C length and the 2nd was a Bb. Cutting all to correct length made a tolerable C instrument.
25 years ago I converted a prewar Besson International Bb to C, I cut 1" off the bell tail, 1" off the leadpipe and 1" off each leg of the tuning slide. My reference was a well tuned piano in church. To get the valve loops in tune I used the instructions in the preface in the Arban Method, play a 4th space E open and 3rd, and adjust the 3rd slide until the notes are the same, play 3rd space C open and 2-3, adjust 2nd slide till notes the same, play 2nd line G open and 1-3 adjust 1st slide until notes are same. While this method may not be perfect it gave me a C trumpet to play in church for some years. My friend played it in orchestra for 3 years with no complaints.
When I did this project I had no knowlege of acoustics or instrument design, only that a C should be about 4" shorter than a Bb, it was a case of Fools rush in.