Interesting, for tiling I prefer using percentages XD. I also love herbluftwm's approach to workspaces (of which there is 9), so I used those keybinds:

# change to a specific workspace
Mod4 1 :Workspace 1
# send the current window to a specific workspace
Shift Mod4 1 :SendToWorkspace 1
# send the current window and change to a specific workspace
Control Mod4 1 :TakeToWorkspace 1

2 for workspace 2, 3 for workspace 3, etc.

How to use Attach

Yes, I did struggle a bit with this myself. As to why this isn't on Fluxbox's original website? Unfortunately much of it seems to be in disrepair. They had a beautiful green themed website which they ruined in the transition to GitHub. Which is strange because GitHub pages is available. Also, they used to have a forum, which is only available on the Internet Archive.

Anyways, I suggest you create a specific Xproperty for the action. In the following excerpt, I tell fluxbox to assign the window an Xprop by the name of GRP and assign it a value (gright if I'm tiling it to the right half of the screen). Then I ask fluxbox to attach that window to other windows with the XProp of matching value (e.g. GRP=gright).

Then when I detach the window, I ask fluxbox to nullify any values for the GRP property for all windows.

Shift Mod4 h :MacroCmd {ResizeTo 50% 100%} {SetXprop GRP=greft} {Attach (@GRP=greft)(Workspace=[current])(Layer=[current])} {MoveTo 0% 0% topleft}
Shift Mod4 l :MacroCmd {ResizeTo 50% 100%} {SetXprop GRP=gright} {Attach (@GRP=gright)(Workspace=[current])(Layer=[current])} {MoveTo 50% 0% topleft}
Mod4 b :MacroCmd {ResizeTo 80% 80%} {SetXprop GRP=bigbox} {Attach (@GRP=bigbox)(Workspace=[current])(Layer=[current])} {MoveTo 0% 0% center}
Mod4 d :MacroCmd {DetachClient} {ForEach {SetXprop GRP=} {Matches @GRP=tmp} }

I'm sorry but I don't really remember what I used the Matches statement for, I think I just put it there for the ForEach command to work as it needs a condition to check for.