First of all, I am French so be indulgent with my English's speaking.
I have to transform a point (in FRANCE) from Lambert 93 to Lambert 1
The point in Lambert 93 is X= 607 067.980m and Y= 7 108 761.646m
Covadis Topo & Circé gives me the same result
X= 553 773.524 and Y = 374 564.154
CoordTrans give me a different result
GRID 1: Lambert 93 (Reseau Geodesique Francais 1993)
GRID 2: NTF France I (NTF)
RESULT: X= 553 118.57 and Y= 137 578.59
I decide to create my own grid (I found these values on Internet) named L1.z1, identical to NTF France I (NTF) except for:
False North = 200 000
SP1 = 48.59852°
SP2 = 50.39591°
The new result is X= 553 775.26 and Y= 374 565.03
It is better but not identical with the two other software's.
I tryed many configurations without success, someone can help me ?