You should be able to easily reach the center of your cutting bed. An ideal width is between 3-4 feet. Below you will find additional link to books, articles, and calculators on cut flower garden spacing.
You can look up the spacing for a particular flower by searching the flower on the Johnny’s website. When you’re on the flowers page scroll down to “Growing Information.” It will be listed under “Plant Spacing.” Ex – https://www.johnnyseeds.com/flowers/zinnias/queen-red-lime-zinnia-seed-1987.11.html – 9-12″
How to space annuals for a cut flower garden – https://www.youtube.com/watch?v=9Vl5Cf0DGDU&feature=emb_logo
Helpful Books that discuss plant spacing –
“Cut Flower Garden” – https://www.amazon.com/dp/1452145768?tag=onamzdaniel0c-20&linkCode=ic5&ascsubtag=amzn1.infl.us.product&creativeASIN=1452145768&ref=exp_northlawnflowerfarm_dp_vv_d
“Cool Flowers” (Only discusses hardy annuals) – https://www.amazon.com/dp/0989268810?tag=onamzdaniel0c-20&linkCode=ic5&ascsubtag=amzn1.infl.us.product&creativeASIN=0989268810&ref=exp_northlawnflowerfarm_dp_vv_d
“Vegetables Love Flower” – https://www.amazon.com/dp/0760357587?tag=onamzdaniel0c-20&linkCode=ic5&ascsubtag=amzn1.infl.us.product&creativeASIN=0760357587&ref=exp_northlawnflowerfarm_dp_vv_d
Planning Calculators – https://www.johnnyseeds.com/growers-library/online-tools-calculators.html
Planning Charts and Articles – https://www.johnnyseeds.com/on/demandware.static/-/Library-Sites-JSSSharedLibrary/default/dweb5dfe61/assets/information/easy-cut-flower-garden-set-planner-map.pdf
https://www.floretflowers.com/how-to-grow-more-flowers-than-you-ever-thought-possible/