If you have done an accurate scale lighting plan on paper using a lighting stencil or a protractor and ruler, you will not have any problems with your stage focusing.
You need a safety plan in place before focusing as the theatre needs to be blacked out for focusing. Make sure you have safe effective backstage working lights covering stage entrances and locations such as stage managers desk. It is a good practice to have a lantern up in an area of stage as a working light when you are not focusing, then take it down when you focus. Explain to everyone present what the term going to lack means so they do not injure themselves in the darkness.
For quick easy focusing have a team of lighting designer, board operator, focusser on the ladder and one or two helpers near the bottom of the ladder as gophers.
When focusing your basic stage areas, start at the end of the bar that has the shortest distance from the lantern to the stage area, as it will have the narrowest beam coverage. Focus at about 90% power so you leave a bit in reserve. Have a person stand in the middle of the stage area to be focused, or in the location of a special so the light can be focused onto the person. Some lighting designers prefer to stand in the focus position with their back to the lantern so their vision is not affected by the light, and will direct the light to where they want it focused.
Once the first lantern is focused at the start of the bar, get the person to move to the edge of the light beam to see if it covers the required stage area. If not adjust the focus, sometimes for a compromise or change lanterns if the beam angle is wrong. Once they have moved to the edge of the lantern coverage, bring up the next lantern, in this case a centre position one, pan the lantern away from the centre of the first lantern until there is a gap, then move the lantern in towards the first lantern until they just overlap. Do not overlap too much as the intensities will not match and you will be wasting beam coverage of the second lantern. Once these two lanterns are focused, repeat the process for the third lantern.