Greetings,
A Month ago I am on the same track and I had actually done a formula.
First find the Men Work required to complete a "Worker Action" on a certain Tile.
MEN WORK = Movement Cost * Base Turn to Complete Action
Both can be found under "Terrain" and under "Worker Action" of "Game Concept" in Civilopedia.
Imagine MEN WORK to be how difficult to do the action on that tile. For example,
mining on mountain = Movement Cost 3 X Action Turn 12 = 36 men work
build fortress on forest = Movement Cost 2 X Action Turn 16 = 32 men work
It is a lot easier to remember the man work required to work on a tile.
then you calculate the worker's work rate
Worker's Rate = Base Worker rate X Industrial Trait Modifier X Government Modifier X Replacement Part Modifier (Round Down the final Worker's Rate)
where
Basic work rate
Foreign worker = 0.5 man work per turn
Local worker = 1 man work per turn
Industrial Trait Modifier
Ind. = 1.5 for C3C, 2 for PTW
non-Ind. = 1
Government Modifier
Anarchy = 1 found in editor
Despotism = 2
Monarchy = 2
Republic = 2
Feudalism = 2
Democracy = 3
Fascism = 4
Communism = 2
Replacement Part Modifier 2 for researched
1 for has not research
A simplier way is to remember the LWWR (Local Worker's Work Rate) which is multiple of all modifiers.
For example, your are industrial democratic in modern age (R.Part researched)
your LWWR is 1.5 x 3 x 2 = 9
your Local Worker will have work rate of 9 men work per turn and your Foreign Worker will have 4 men work per turn (half of 9 is 4.5, round down becomes 4)
Turn to Complete = Men Work / Worker's Work Rate (roundup)
If you want to Build fortress on forest (which is 32 men work), you need :
Local Foreign
4 0 = 4*9 + 0*4 = 36 + 0 = 36 men work or
3 2 = 3*9 + 2*4 = 27 + 8 = 35 men work or
2 4 = 2*9 + 4*4 = 18 + 16 = 34 men work or
1 6 = 1*9 + 6*4 = 9 + 24 = 33 men work or
0 8 = 0*9 + 8*4 = 0 + 32 = 32 men work (best combination)
I had done an excel worksheet to help the calculation.
www.civfanatics.net/uploads7/Men_Work.zip
If you need further explaination, I'm most willing to help
Regards