Assuming ambient is sufficient to register on the ISO100 film, and to use your Metz as fill...

1. Set Auto (flash photosensor) mode, aim Metz flash head up to ceiling...photosensor takes care of compensating for light loss due to bounce
2. but set ISO on the 45CL4 to FASTER than actual film speed (e.g. set ISO 400 on flash will result in -2EV fill)

Easy!

If ambient is too low to register 'proper' exposure, ...
1. if possible use an aperture+shutter combination which will register ambient light about a -1EV below 'proper', and use the flash as primary illumination.
2. Set Metz to actual film ISO100, use Auto mode and flash head aimed up to ceiling to bounce