The splitting behaviour is controlled by display-buffer-alist. You can customize this variable to get the desired splitting.

Please refer https://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Action-Functions.html

In particular, note display-buffer-below-selected.

An easier to achieve the same is using the package shackle (my personal choice)

For example, the below code aligns all helm buffers to the bottom of the screen, covering 40% of the screen

(setq shackle-rules '(("\\`\\*helm.*?\\*\\'" :align 'below :size 0.4)))

You could do the same for buffers with a given mode, try something like

(setq shackle-rules '((slime-mode :align 'below)))

The splitting behavior can be controlled by setting the variable display-buffer-alist.

Please refer https://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Action-Functions.html

In particular, note display-buffer-below-selected.

An easier to achieve the same is using the package shackle (my personal choice)

For example, the below code aligns all helm buffers to the bottom of the screen, covering 40% of the screen

(setq shackle-rules '(("\\`\\*helm.*?\\*\\'" :align 'below :size 0.4)))

You could do the same for buffers with a given mode, try something like

(setq shackle-rules '((slime-mode :align 'below)))