Baffle Placement for a Refractor
Minimum Baffle Method
 Baffle Placement Illustration #1

First, draft out a scale drawing (full size is better) of your proposed scope. Include all items which limit the light path such as the focuser tube lens cell. Make sure to account for the focuser tube's extension into the tube when the image is at focus. Also, don't forget the star diagonal if it will be used.

Select a desired size for the illuminated field and draw it at the focal plane. Draw the light cone by drawing a line from each edge to the respective edge of the objective.
 Baffle Placement Illustration #2
Next, draw a line (red, above) from the bottom edge of the field spot to the tube wall at the back edge of the objective. Where this line crosses the light cone is the first baffle position.
 Baffle Placement Illustration #3
Now draw a line (green in the illustration) from the bottom of the objective to the first baffle position and extend it to the tube wall.
 Baffle Placement Illustration #4
Draw a line (red) from the bottom of the field spot to the place where the previous line hits the tube wall. Where this new line intersects the light cone is the position of the second baffle.
 Baffle Placement Illustration #5

Do the same thing repeatedly until you run out of room. In most small refractors, two or three baffles will be all that is needed. In the Okapi, only two were used because the focuser tube acts as the third one.

NOTE: Using this method makes portions of the tube wall visible to the field spot, but only those portions shaded by the nearest baffle. Thus, flat black paint is necessary. In Okapi, the entire interior of the tube is ridged with small cuts to act as mini-baffles down the length of the tube.

If you want to make sure no tube wall is visible, at the expense of perhaps using more baffles, skip the green lines from the objective and draw the red lines to the base of each baffle itself.