I think you can only find the speed if you set that slide block to move a certain number of pixels (so make sure you have Slide self BY <number_of_pixels>) and you have no effect on it (like Back In/Out, Strong In/Out or any other options in that drop down menu ). The rest is like you said...the time you have it set in the block, and also the space is the number of pixels you have in there.
Do keep in mind that if you do use the effects in that drop down menu you're adding acceleration into the mix and the math changes a little (actually it's only very hard to calculate if the acceleration is not constant), and if you plan on using the slide bloc without those effects you're basically achieving the same thing as using Set x-speed of self.