Exposure when projecting with an enlarger is fairly similar to the exposure when contacting with the same enlarger setup (less negative) as a light source. Therefore, measuring or guessing at the exposure, and changing it in proportion to the sensitivity of the film will give accurate enough exposure for the first test shot. Another technique for making transparencies is to shoot them in a macro camera. Exposure is calculated much like any other macro shot. One problem with making B&W transparencies: most film yields low contrast transparencies. Tech Pan developed in print developer was an exception. I haven't tried any current high contrast films. Extended development of normal films may boost contrast to practical levels, but I haven't had to try this.